Ankylosing spondylitis (AS) is a chronic disease that develops slowly. In the early stages of the disease, the sacroiliac joints (located at the back of the pelvis) become inflamed and painful. In fact, one of the earliest signs of AS is tenderness around the sacroiliac joints. Another early symptom is low back pain that may spread down into the buttocks and thighs. Pain varies in intensity and duration, and it is episodic (comes and goes). Stiffness is usually worse in the morning and improves with exercise.
As the disease progresses, ossification (new bone growth) is triggered by the body's defense mechanism. Your body knows that joint movement is causing pain, so it attempts to limit movement by forming new bone that will stop movement. Ossification causes new bone to grow between vertebrae, eventually fusing them together and increasing the risk for fracture. Furthermore, ossification may affect spinal ligaments, causing spinal stenosis (narrowing of nerve passageways, spinal canal), which can result in neurologic deficit (problems related to the nerves, such as weakness or loss of function).
Other symptoms of ankylosing spondylitis: